1. Field of the Invention
A device is disclosed for measuring the thickness of a layer of nonconducting material during the movement of the material from one location to another. The nonconducting material serves as a dielectric sandwiched between two plates of a capacitance detection probe. A material thickness induced variation in the dielectric between the two electrically conducting plates generates a frequency output alteration in a plate associated oscillator. Storage and analysis of the frequency alteration information is employed to generate a thickness measurement for the nonconducting material.
2. Description of the Background Art
Various devices and methods have been developed to detect the thickness of materials passing a measurement detection point at low to high speeds. Many of these procedures were directed to determining thickness variations in a continuous piece of thread or yarn. Other methods were used with continuous sheets of materials. Some attempted to record the passage of turbine blades at high velocities. These approaches generally rely on detecting a signal generated by the material passing near a detection element, but are often subject to numerous errors due to uncorrected, uncorrectable, or improperly corrected signal generation and analysis.
Both electrically conducting and nonconducting materials have had their thicknesses measured by various techiques, including rudimentary capacitance detectors. However, due to inherent deficiencies in these prior capacitance techniques, variations in surrounding air, machine, and measured material temperatures, machine vibrations, random electronic drift, and like sources of error were not easily corrected or even noticed and introduced inaccuracy in the determined material thicknesses. The subject device and method of use overcome these difficulties.
Disclosed in U.S. Pat. No. 3,221,171 is a method and apparatus for measuring the denier or fineness of yarn or thread using photosensitive or capacitive means. The yarn forms part of the dielectric between detection plates. Simplistic circuits produce measurable voltage fluctuations upon yarn thickness variations.
U.S. Pat. No. 3,341,774 relates a capacitance detector having a transmitter connected to one plate and a receiver connected to another plate. This device eliminates the grounding for one of the detection plates and indicates the drawbacks surrounding grounded detectors.
Delineated in U.S. Pat. No. 3,471,780 is a capacitance thickness gauge comprised of a charged sensing plate sandwiched between primary and secondary grounded plates. The technique claims to correct for moisture content in the measured material and atmosphere. A capacitance meter is employed to monitor variations in the dielectric.
Described in U.S. Pat. No. 3,519,922 is an apparatus for detecting abrupt changes in the thickness of sheet materials. Two sequentially spaced capacitance detectors having an even thickness material passing between them produce similar capacitances. However, if a sudden material thickness change occurs, a difference in dielectric between the two capacitance detectors is noted.
U.S. Pat. No. 3,523,246 shows a method and apparatus for testing a laminar material for irregularities of thickness. The tested laminar material must be at least partly conductive of electricity. An alternating potential difference is applied to the material to be tested between two positions. Recorded potential variations between these two positions reflect thickness deviations.
Presented in U.S. Pat. No. 4,006,411 is a static capacitance type sensor for detecting yarn denier. When yarn is passed through a sensing condenser, the static capacitance is varied and the circuit deviates from the series-resonant position and a variable input will appear at the input terminal of an amplifier.
A blade tip clearance measuring apparatus is shown in U.S. Pat. No. 4,063,167. Rotating turbine blade tips alter the capacitance of a detection probe as they pass nearby. Also, rotating blades are detected by a capacitance sensor described in U.S. Pat. No. 4,071,820. The '820 patent illustrates the general principle of employing a non-contacting sensor that is responsive to an energy manifestation as a function of the spacing between the sensor and the object being measured.
U.S. Pat. No. 4,086,528 discloses a capacitance detector system having one capacitance transducer with at least one capacitor that varies in capacitance in accordance with changes in the dimensions of the quantity being measures and one reference capacitor.
A calibrating and measuring circuit for a capacitive probe type instrument is depicted in U.S. Pat. No. 4,130,796. This device is for gauging the texture of a conducting surface and contacts with the surface to be measured.
U.S. Pat. No. 4,208,625 reveals a capacitive measuring system with automatic calibration. The automatic calibration stems from a comparison routine between an absolute measurement for say the denier of a yarn and a prescribed datum. Continuous monitoring of the characteristics of a moving filament is achieved. Grounding is not associated with the detection capacitor plates since a sensor head forming a capacitance bridge is employed and driven by a signal generator.
The size and shape of a body is determined by the device related in U.S. Pat. No. 4,284,947. The body to be measured is rolled between the plates of capacitors, the dielectric constant of which is thereby altered.
Shown in U.S. Pat. No. 4,311,958 is a capacitive thread motion detector and thread motion stopping device. An alternating current signal is generated during the passage of a thread, with inherent physical irregularities, through a capacitive element, but when the thread breaks or stops the signal disappears and the associated machine is halted.
U.S. Pat. No. 4,706,014 discloses an improved device for measuring the diameter of a dielectric fiber. Key to this invention is the particular shaping of the capacitor plates, such as to allow electric field reductions at the edges to be compensated for and hence capacitance measurements to be unaffected by fiber vibrations.
A capacitive cell for continuous measurement of the linear mass of textile products is described in U.S. Pat. No. 4,710,701. Two identical air capacitors are employed in this device. One capacitor has air as the dielectric and the other has a continuous moving thread. Thread irregularities are detected by comparing capacitance differences between the two capacitors.